Whenever it is desired to close a breaker connecting two a.c. power systems, the system voltages must be in synchronism at the time of closing to prevent serious equipment damage to either of the systems or the breaker from abnormal power flow. To prevent a breaker from closing under unsynchronized conditions, a Synchronism Check Relay is used. In most cases the relay will not close the breaker directly but will close relay contacts which will permit an operator to close the breaker. Even at large slip frequencies the two voltages are in synchronism for short periods of time during each cycle of the slower frequency. As the slip frequency decreases the aforementioned time interval of synchronism increases. In order to be able to close the breaker at a desired slip frequency, a time delay is incorporated. Since the length of the time delay is a function of the frequency difference between the two signals, the longer the time delay, the closer to equal the frequencies must be before the two lines are closed (i.e. connected).
The term synchronism refers to the relationship between the magnitude and phase of each system. Two voltages are in synchronism when both their magnitudes and their phase angles are within preset limits.